1. Field of the Invention
The present invention relates to a heat treatment method for a silicon monocrystal wafer obtained by slicing a silicon monocrystal ingot grown by a Czochralski method (hereinafter referred to as xe2x80x9cCZ methodxe2x80x9d), and a silicon monocrystal wafer wherein crystal defect density is significantly reduced by the method.
2. Description of the Related Art
As a method for eliminating crystal defects of a silicon monocrystal wafer, there has been adopted a method in which a wafer is subjected to hydrogen anneal at a high temperature. In this method, oxide precipitate is intentionally reduced with hydrogen and dissolved, and thus surface oxide-film is eliminated, resulting in increasing oxygen out-diffusion speed, so that defects can be eliminated. However, it is known that even if the hydrogen anneal is performed under the typical condition that a temperature is 1200xc2x0 C. and treatment time is 60 minutes, since crystal defects and oxide precipitates remain near the surface of the wafer, oxygen is continuously out-diffused from inside to outside, and oxide precipitates are re-formed. Further, this method strongly depends on history of crystal before hydrogen anneal treatment, and therefore the wafer having less crystal defects has been selected as a wafer which is to be subjected to hydrogen anneal treatment.
As an another method, there has been adopted a method in which crystal defects are eliminated by lowering a growth rate of monocrystal. However, according to this method, although the number of crystal defects can be decreased, electronic characteristics such as oxide dielectric breakdown voltage of a device are degraded since the size of the defect is increased.
As described above, it is difficult to eliminate crystal defects sufficiently, even when the silicon monocrystal wafer is subjected to hydrogen anneal. Furthermore, lowering the pulling rate results in increase of the defect size, and therefore it is difficult to eliminate defects with hydrogen anneal.
Meanwhile, COPs (Crystal Originated Particles) have recently been cited as a cause of decreasing the yield of a device-fabricating process. COP is one type of crystal defects that are introduced in a crystal during the growth thereof and is known to be a defect of vacancy type having a regular octahedron structure void or cavity.
When a silicon wafer having COPs subjected to mirror-polishing is cleaned through use of a mixture solution of ammonia and hydrogen peroxide, pits are formed in the wafer surface. When the number of particles on the wafer is measured through use of a particle counter, pits are also detected and counted as particles together with real particles. The thus-detected pits are called xe2x80x9cCOPsxe2x80x9d in order to distinguish them from the actual particles.
COPs existing at the surface layer portion of a wafer degrade the electric characteristics of the wafer. For example, a time dependent dielectric breakdown (TDDB) of oxide film, one important electric characteristic of a semiconductor device determined through a reliability test, is known to be related to COPs, and therefore reduction of COPs is required in order to improved the time dependent dielectric breakdown.
Also, COPs are said to affect an ordinary time zero dielectric breakdown (TZDB) of oxide film.
Moreover, COPs are said to adversely affect the device-fabricating process. For example, if COPs exist at the surface layer portion of a SOI (Silicon On Insulator) wafer, buried oxide film is etched by etchant or atmosphere gas passed through the COPs during an etching process or a heat treatment process, and a step is formed during a wiring process, and the thus-formed step causes breakage of wiring, resulting in a decrease in yield.
A hydrogen anneal is known as a method for reducing the COPs. However, even if anneal is conducted under the typical treatment condition, COPs at the surface layer portion of the wafer can not be completely eliminated, but partly remain. Furthermore, COPs also remain at relatively near area to the surface.
The reason of why COPs at the surface layer portion of the wafer can not be completely eliminated is, for example, that COPs remain in the wafer even when high temperature hydrogen anneal is conducted at 1200xc2x0 C., for 60 minutes, and later, internal COPs come to appear on the surface as a result of that the surface is etched during hydrogen anneal. COPs which come to appear on the surface right before the temperature begins to be lowered are difficult to be eliminated while the temperature is falling, and the COPs which come to appear on the surface while the temperature is falling are more difficult to be eliminated. In order to prevent COPs from appearing while the temperature is falling, it is necessary to raise the temperature falling rate.
Because, silicon is generally etched in a thickness of about 0.5 xcexcm through hydrogen anneal at 1200xc2x0 C. for 60 minutes, and etching rate decreases and migration on the surface of the silicon get small, as temperature lowers. And therefore, COP appearing on the surface while the temperature is lowering is not etched, and is difficult to be eliminated.
Alternatively, in order to reduce COPs, it is necessary to prepare a wafer having COPs which can be easily eliminated by hydrogen anneal, and for that purpose, it is necessary to study thoroughly the condition for pulling silicon monocrystal ingot which is to be used for preparation of a wafer. In the prior art, a growth rate of monocrystal is lowered in order to reduce defects such as COP. However, in this case, although the number of COP can be decreased, the size thereof gets large, and therefore, the probability that COPs are not eliminated is high even if the wafer prepared from the monocrystal ingot thus obtained is subjected to hydrogen anneal. Therefore, it is difficult to eliminate COP defects by the existing technique.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to manufacture a silicon monocrystal wafer in which crystal defects existing on the surface of the wafer or at the surface layer portion thereof are minimized, and to provide a silicon monocrystal wafer for a device which is excellent in not only oxide dielectric breakdown voltage but also other electric characteristics such as electrical reliability.
Another object of the present invention is to enable production of a silicon monocrystal wafer having no defect, and to achieve enhancement of productivity, reduction in the amount of hydrogen to be used, cost reduction, and the like.
To achieve the above object, the present invention relates to a heat treatment method for a silicon monocrystal wafer comprising the steps of heat-treating in a reducing atmosphere a silicon monocrystal wafer manufactured by slicing a silicon monocrystal ingot which is grown by Czochralski method wherein a wafer obtained by slicing a silicon monocrystal ingot having oxygen concentration of 16 ppma or less which is manufactured by pulling at a growth rate of 0.6 mm/min or more, and in which COPs exist in high density is subjected to anneal heat treatment at 1200xc2x0 C. or above for one second or more through use of a rapid heating/rapid cooling apparatus.
The rapid heating/rapid cooling means, for example, a method that a wafer is immediately loaded into a heat treatment furnace in which a temperature is arranged in the above-mentioned range, and is immediately loaded out upon elapse of the above-mentioned heat treatment time, or a method that a wafer is immediately subjected to heat treatment with a lump heater or the like after it is arranged at a predetermined position in the heat treatment furnace. The language reading xe2x80x9cimmediately loaded intoxe2x80x9d or xe2x80x9cimmediately loaded outxe2x80x9d means that there are not performed an operation for raising and lowering the temperature over a certain period, as well as a conventional so-called loading or unloading operation in which a wafer is slowly loaded into the heat treatment furnace and slowly loaded out. Of course, it takes a certain time to bring a wafer to a predetermined position in the furnace, for example, several seconds to several minutes depending on capability of a transfer apparatus for loading of a wafer. The apparatus having the above-mentioned function is called a rapid thermal annealer (hereinafter abbreviated to xe2x80x9cRTA apparatusxe2x80x9d).
As described above, a silicon monocrystal ingot having an oxygen concentration of 16 ppma or less in which COPs exist in high density is manufactured by pulling a silicon monocrystal ingot by CZ method at a high growth rate of 0.6 mm/min or more, preferably 0.8 mm/min or more. Then, the wafer obtained by slicing the silicon monocrystal ingot thus manufactured is subjected to anneal heat treatment with a rapid heating/rapid cooling apparatus in a reducing atmosphere at a temperature of 1200xc2x0 C. or above for one second or more, so that there can be obtained the wafer in which COPs on the surface and at the surface layer portion thereof are significantly reduced. Accordingly, there can be obtained a device that not only oxide dielectric breakdown voltage but also other electric characteristics such as electrical reliability are significantly improved. Furthermore, a wafer having a large diameter in which COPs is especially difficult to be reduced can be treated in a short time, and therefore, enhancement of productivity is achieved, and safety is improved since an amount of hydrogen gas to be used can be decreased.
The present invention also relates to a heat treatment method for a silicon monocrystal wafer comprising the steps of heat-treating in a reducing atmosphere a silicon monocrystal wafer manufactured by slicing a silicon monocrystal ingot which is grown by Czochralski method wherein a wafer obtained by slicing a silicon monocrystal ingot having oxygen concentration of 16 ppma or less which is grown at a growth rate of 0.6 mm/min or more, preferably 0.8 mm/min or more, and in which COPs exist in high density is subjected to anneal heat treatment at 1200xc2x0 C. or above for 30 minutes or more through use of a batchwise heat treatment furnace.
A batchwise heat treatment furnace means a furnace in which a heat treatment is performed in a so-called batchwise operation, that is, plural wafers are placed on plural shelves provided in the vertical type heat treatment furnace, hydrogen gas is then introduced therein, temperature in the furnace is raised relatively slowly, and then heat treatment is performed at a predetermined temperature for a predetermined period, followed by lowering the temperature relatively slowly. When using the batchwise furnace, a large amount of wafers can be subjected to heat treatment. However, one cycle additionally including time for loading in and loading out of the wafer is long, and therefore productivity is not so excellent as that of RTA apparatus. However, it is excellent in controllability of temperature so that stable operation can be achieved.
When the wafer manufactured from the silicon monocrystal ingot having the similar quality to one of the ingot used in the above described method using a rapid heating/rapid cooling apparatus is subjected to anneal heat treatment using the batchwise heat treatment furnace at the temperature of 1200xc2x0 C. or above, for 30 minutes or more, COPs on the surface and at the surface layer portion thereof are significantly reduced, electric characteristics of the device such as oxide dielectric breakdown voltage, electrical reliability or the like are significantly improved. Furthermore, as for a wafer having large diameter, enhancement of productivity and cost reduction are achieved.
In this case, COPs existing in high density in a silicon monocrystal to be subjected to the heat treatment preferably have a size of 60-130 nm, and preferably consist of only one void.
The silicon monocrystal in which such a fine COPs exist in high density can be easily manufactured by pulling at 0.6 mm/min or more, preferably 0.8 mm/min or more. Furthermore, when monocrystal has oxygen concentration of 16 ppma or less, almost no oxide film exists in the inner wall of COP, and therefore, COPs can be eliminated quite easily with hydrogen anneal heat treatment of the wafer.
Furthermore, oxygen concentration in the monocrystal can be controlled by adjusting a rotating number of a crucible containing silicon melt therein as a raw material, a rotating number of a growing monocrystal, quantity of flow of inert gas, temperature of melt, or the like.
The present invention also relates to a heat treatment method of a silicon monocrystal wafer wherein the silicon wafer having COPs of a size of 60-130 nm is subjected to heat treatment in a reducing atmosphere at a temperature of 1200xc2x0 C. or above. Furthermore, the present invention relates to a heat treatment method of a silicon monocrystal wafer wherein the silicon wafer wherein COP is crystal defect consisting of only one void is subjected to a heat treatment in a reducing atmosphere at a temperature of 1200xc2x0 C. or above.
Such a fine COP, or COP which is crystal defect consisting of only one void can be easily eliminated by high temperature heat treatment in a reducing atmosphere, regardless of a method for manufacturing a wafer.
In an embodiment, when the above mentioned reducing atmosphere is 100%-hydrogen atmosphere, or a mixed atmosphere of hydrogen and argon, sufficient effect of hydrogen anneal heat treatment can be obtained, COPs having oxide film on the inner wall can be significantly decreased, and a void can be filled with silicon, so that almost no defect wafer can be provided.
Furthermore, the present invention relates to a silicon monocrystal wafer in which COPs are eliminated by the method described above.
In the silicon monocrystal wafer manufactured by the heat treatment method described above, since the size of COP is small, oxide film on the inner wall of COP can be reduced and dissolved by hydrogen surely diffused from the surface of the wafer thereto by hydrogen anneal, a void can be filled with silicon supplied from the surface of the wafer to be eliminated, and thus COPs can be eliminated, so that actually no defect silicon monocrystal wafer can be provided. Accordingly, characteristics of a device is improved, yield is improved, so that quite useful silicon monocrystal wafer can be provided.
According to the present invention, a wafer obtained by slicing a silicon monocrystal ingot having oxygen concentration of 16 ppma or less which is manufactured by pulling at a growth rate of 0.6 mm/min or more, and in which COPs exist in high density is subjected to high temperature heat treatment in a reducing atmosphere through use of a rapid heating/rapid cooling apparatus or a batchwise heat treatment furnace, so that COPs on the surface and at the surface layer portion can be significantly reduced and thus the no defect wafer can be manufactured. Accordingly, the wafer is quite valuable as a wafer for a device which is excellent in electrical characteristics. Furthermore, especially for a large diameter wafer, since high speed pulling can be performed, it is possible to achieve enhancement of productivity and cost reduction by selecting the monocrystal growing condition and hydrogen anneal condition appropriately. Moreover, reduction in the amount of hydrogen to be used can be achieved.